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Zinc is not uncommon in chromites and other spinels, especially in
hydrothermally altered rocks. There is actually a Zn endmember:
zincochromite (Nesterov & Rumjantzeva, American Mineralogist, volume  73,
pp. 931-932 (1988)

In systems with high Zn/S, large amounts of Zn will go into many other
ferromagnesian minerals, including micas, ilmenite, rhodonite, clays and
pyroxenes.

See also :
Wylie, A.G., Candela, P.A. and Burke, T.M. (1987) Compositional zoning in
unusual zinc-rich chromite from the Sykesville District, Carroll Co.,
Maryland. American Mineralogist, v. 72, p. 413-422.

Ralph Bottrill
        -----Original Message-----
        From:   Kevin Leahy [SMTP:[log in to unmask]]
        Sent:   Friday, May 11, 2001 12:07 AM
        To:     [log in to unmask]
        Subject:        Zn metasomatism and chromite

        Dear All,

        I am currently working on a diamond exploration project where the
only
        indicator minerals (of kimberlite and lamproite) turning up in the
        tropically-weathered soils are chromites - this is not so unusual
because
        the silicates often get toasted by the laterisation.

        However, the chromites are extremely Zn rich (for chromite anyway)
around
        10% ZnO, with MgO <2%, and Cr2O3 51-55%. Has anyone else ever come
across
        this type of chromite? Is it likely to be mantle metasomatism? can
there be
        a crustal paragenesis?

        More generally, has anyone any information on the replacement of Mg
with Zn
        in any other type of mineral, in a lithospheric mantle (or any
other)
        setting?

        Thanks
        Dr Kevin Leahy

        Exploration Consultants Ltd
        Henley-on-Thames
        UK